This invention pertains to the field of high performance sailboats supported by either flotation or by hydrodynamic forces or by a combination of both.
In general, a hydrofoil sailboat, when compared to a conventional catamaran, has superior performance in strong winds and inferior performance in light winds. While a hydrofoil sailboat can have superior performance when on a broad reach, it suffers from a performance inferior to conventional catamarans when sailing close to the wind. The larger catamarans have a marked superior performance over the smaller sizes but are not as easily trailered or handled and are considerably more expensive. Smaller hydrofoil sailboats are not so limited in high performance and offer the fascination of being able to take off and become foil borne. The potential of combining the best features of hydrofoil and multihull sailboats has not been fully realized.
A sailboat must develop an underwater resistance to the side force caused by sail wind pressure. This is usually accomplished by the leeway action of the boat which causes pressure differentials on the underwater surfaces of the hull. The drag on the boat exerted by the underwater pressures is dependent largely upon the aspect ratio of the underwater surfaces. The aspect ratio is measured by the average draft divided by the water line length of the boat. The aspect ratio is improved by the addition of such as centerboards, dagger boards or keels. However, these surfaces, as customarily used, fall short of fully minimizing the underwater drag due to side forces.
A sailboat's thrust is derived from wind forces acting collectively at a point approximately at the geometric center of the sails. This sail force is divided into two components: the heeling force which acts in a beamwise direction toward the lee side of the boat and the pitching force which acts in a forward direction parallel to the length of the boat. The heeling moment is a couple consisting of the heeling force and an equal and opposite force exerted against the boat's underwater surfaces in a beamwise direction. The pitching moment is a couple consisting of the pitching force and an equal and opposite force resisting the forward motion of the boat in the water. Obviously, the pitching force is instrumental in producing forward motion and the heeling force produces leeway. The pitching moment tends to force the bow down and, in extreme cases, can cause pitch poling which is a capsize with the boat stern rotating up over the bow. The heeling force tends to cause the boat to lean in a beamwise direction where the sails are considerably less efficient and, in extreme cases, roll the boat over to where it capsizes. In high speed sailboats, a capsize is usually a combination of heeling and pitching. The speed of a sailboat is obviously greatly effected by the pitching and heeling force and moment.
A sailboat's maximum speed on any tack is limited by a number of factors one of which is the maximum permissible sail force that can be sustained without capsizing by heeling, by pitch poling or by a combination of both. Stability of conventional sailing craft is achieved by advantageous separation of the center of gravity of the boat and the center of buoyancy (or center of support by hydrodynamic forces). Conventional attempts to improve heeling stability have lead to such devices as outriggers and trapezes to support the crew while suspended over the windward side to increase the separation between the center of buoyancy and center of gravity. The use of hydrodynamic forces to counter heeling tendency has not been fully exploited.
High speed catamarans suffer from a tendency toward bow down attitude bordering on pitch poling even with the crew as far aft as possible. When a sailboat is in a bow down attitude, mild wave action can induce pitch poling. Also, the hull lines created by the bow down attitude offer inferior hydrodynamic characteristics. The use of hydrodynamic forces to produce antipitching moments has not been fully realized.